1. Field of the Invention
This invention relates to aqueous slurries and particularly relates to non-settling and flowable aqueous slurries of sodium dithionite that remain in pumpable form without significant expansion, settling, or gellation.
2. Review of the Prior Art
Sodium dithionite, commonly termed sodium hydrosulfite, and, less correctly, sodium hyposulfite, is a powerful reducing agent that has long been used for bleaching, particularly for bleaching textiles and wood pulps such as ground wood and semi-chemical pulps.
Sodium dithionite has usually been manufactured by significantly different processes that are alternatively based on zinc dust, sodium formate, sodium borohydride, or sodium bisulfite (electrolytic). Such processes are disclosed, for example, in U.S. Pat. Nos. 2,938,771, 3,004,825, 3,259,457, 3,411,875, 3,718,732, 3,872,221, 3,887,695, 3,897,544, 3,927,190, and 4,127,642.
The products of these processes are herein respectively identified as zinc-derived, formate-derived, borohydride-derived, and electrolytically-derived sodium dithionite. Because the zinc process produces zinc dithionite which is no longer ecologically acceptable, zinc dithionite is converted to sodium dithionite by adding sodium hydroxide or sodium carbonate, whereby zinc hydroxide or zinc carbonate is precipitated and removed by filtering.
Sodium dithionite from any of these four processes is potentially available on a commercial basis in the form of crystalline powders, solutions, or slurries. When sodium dithionite is prepared under conditions which are controlled so as to grow elongated crystals of a size which can be readily separated from the mother liquor by sedimentation or filtration, suitable crystals have a length of at least 320 microns and a thickness of at least 120 microns. If allowed to settle as an aqueous sodium dithionite slurry, a cake is formed under the supernatant liquor which is quite dense and hard packed; it even solidifies in time to a form resembling concrete. Although the supernatant liquor protects the cake from decomposition, the cake is obviously not a useful form of the dithionite.
When anhydrous sodium dithionite crystals are dissolved under either aerobic or anaerobic conditions to make a large quantity of aqueous solution, the resulting solution cannot be stored for use over a long period of time. Due to hydrolytic decomposition at the natural pH of the sodium dithionite solution, decomposition will proceed rapidly from that point by self-propagation because the decomposition products create an acidic condition which accelerates the decomposition.
Aqueous solutions of the dithionite will decompose at a commercially tolerable rate, however, if stabilized by additives such as are disclosed in U.S. Pat. Nos. 3,819,807 and 3,985,674. These additives include chelating agents, sodium carbonate, sodium tripolyphosphate, sodium hydroxide, and amines.
Although such stabilized solutions can be protected from decomposition for long enough periods for shipment and routine commercial use under suitable conditions, it has been more common practice to store the anhydrous dithionite crystals under a dry, inert gas in a sealed container. Even though the crystals are thereby chemically stable for long periods, they begin to decompose as soon as exposed to the air and moisture when the container is opened for use thereof.
Furthermore, commercially available solutions of sodium dithionite are expensive to transport because they are typically at concentrations of 12-13.5%, when combined with suitable additives, and additionally generally require refrigeration during shipment and storage. Thus, the transport of about seven times as much water as product tends to cause the sale of this commodity to become distance-dependent. In consequence, slurries have seemed to offer an inviting means to avoid or at least to minimize the cost of storage and difficulties associated with solution forms of sodium dithionite, without decreasing the convenience that the purchaser derives from solutions.
However, the economical preparation, stabilization, handling, and shipping of such slurries is not simple. Adequate suspension without agitation, so that pumping can be done from a tank truck after shipment, is also not easy. In fact, after considering the variety of processes that are available for manufacturing sodium dithionite, including the indigenous by-products, crystal structures, and the like, the complexities of the concept are readily appreciated. Moreover, slurries have not been as widely investigated nor as commercially utilized as other forms of sodium dithionite.
U.S. Pat. No. 3,536,445 describes a process for making sodium dithionite from sodium-zinc alloy by initially producing zinc dithionite and then converting it to sodium dithionite by adding caustic soda. After removal of the zinc hydroxide by filtration, the dihydrate of sodium dithionite is "salted out" of the mother liquor with sodium chloride and alcohol to form a slurry.
U.S. Pat. No. 3,804,944 gives some stability storage data for 30% slurries (18.5% formate-derived and 11.5% zinc-derived sodium dithionite) containing 1-8% caustic soda (dithionite basis). Tests showed that these slurries required frequent agitation to prevent caking and handling difficulties.
U.S. Pat. No. 3,839,217 shows that by reducing the particle size of the sodium dithionite crystals and/or introducing a suspending or thickening agent into a liquid containing the crystals, such as alcoholic brine, it is possible to form a fluent, homogeneous, pourable dispersion of the solid dithionite particles which is chemically and physically stable for long periods of time, provided that a material, such as the salt in the brine and/or an alcohol, be present which suppresses the dissolution of the dithionite, so that the dispersion can be stored at about 20.degree. C. The majority of the particles should be about 0.6-0.8 micron in size. Methylcellulose, hydroxyethyl cellulose, polyvinyl alcohol, quar gum, and other common thickening, dispersing, or suspending agents can be used. The thickened dispersion exemplarily has a Brookfield viscosity of 9,000 cps and contains up to 34% Na.sub.2 S.sub.2 O.sub.4.
U.S. Pat. No. 3,839,218 provides a method for maintaining a dispersion of crystalline zinc or alkali metal dithionite hydrate by continuous or periodic mechanical agitation so that the crystals can be stored for long periods without decomposition, the dispersing medium being aqueous or non-aqueous and containing a material which suppresses dissolution of the dithionite solids. The pH of the liquid must be at least 6.5, the viscosity of the dispersion must be below about 50,000 centipoises, and the suppressing material may be a water-soluble organic compound or a saturated brine or mixtures thereof. A thickening and suspending agent can be used. Suitable agents include polysaccharides, water-soluble polymers, and proteins of moderate molecular weight. Exemplary agents include guar gum, gum tragacanth, gelatin, and starch.
U.S. Pat. No. 4,283,303 discloses a method for making substantially stable slurries containing 30-35% by weight of sodium dithionite by evaporating sodium dithionite solutions while maintaining the heating medium at 220.degree.-250.degree. F. and the solution and slurry at 110.degree.-155.degree. F. under a vacuum of at least 25 inches Hg and by promptly cooling the resultant slurry while agitating it. The vacuum is preferably 26.5-27.5 inches Hg. Zinc-derived sodiation liquor is the preferred sodium dithionite solution to which 4-5% by weight of the sodium dithionite, NaOH, and a chelator, as a stabilizing agent, are added.
Although these evaporated slurries have excellent stabilization qualities, they have developed problems with settling which has occurred over a period of 2-5 days and especially under the vibrations produced by tank car shipment. Such settling and subsequent hardening has resulted in shipments which could not be unloaded by pumping as would normally be done.
Slurries are utilized as foods, coatings, paints, dyes, explosives, oilwell fluids, and the like and often include natural or synthetic gums to form a liquid colloidal system in which the solid particles are dispersed. Such a gum containing fluid system, without the solid particles, is identified as a sol and is more accurately termed a hydrosol when based on water.
The gums typically impart viscosity to sols in which they are incorporated and thereby function as thickeners. When shear forces are created by agitating a sol and there is no change in viscosity, the behavior of a thickener is said to be Newtonian. When the viscosity of the sol in a quiescent state is greater than when a shear force is applied through agitation, when the viscosity decreases as the appliced shear force increases, and when the viscosity recovers immediately when the magnitude of the shear force is decreased, the behavior of a thickener is said to be plastic. When the rate of flow increases faster than normally in relation to the applied shearing stress, the sol is described as pseudoplastic. Generally, when the sol is at rest, the molecules of a plastic thickener arrange themselves into a more or less stable form. In order to break this stable molecular arrangement and cause the sol to yield, the application of a shear force is necessary. The shear force that is required to cause the sol to yield and flow is termed the yield point or the gel strength. Once the gel strength of a plastic sol is overcome, the viscosity of the sol proportionately decreases as greater shear force is applied.
Numerous natural and synthetic gums are widely used for manufacturing hydrosols. Favored gums for many hydrosols are galactomannan gums such as guar gum, which is derived from the endosperm of the guar plant, Cyamopsis tetragonolobus. Other water-soluble gums which are increasingly utilized are the xanthonomas hydrophilic colloids, commonly termed xanthan gums, which may be produced by the action of various bacterial species of the genus Xanthomonas on carbohydrates (and like materials). The fermentation product of the reaction of the bacteria Xanthomonas campestris, a preferred species, on carbohydrates is commercially available as "Kelzan XC Polymer", made by Kelco Corporation of San Diego, Calif.
In a typical process for clarification of a xanthomonas fermentation broth and/or recovery of the xanthomonas hydrocolloid component, the broth is diluted with water to reduce its viscosity, and optionally the diluted broth is centrifuged or filtered to remove suspended insoluble solids. A salt such as potassium chloride and a nonsolvent such as methanol or isopropanol are added to the broth to flocculate the gum in the potassium form, which gum is then recovered by centrifugation or other solid/liquid separation technique. Further dissolving, reprecipitating, and washing steps are usually employed. The heteropolysaccharides thus prepared by bacteria of the genus Xanthomonas on carbohydrates are normally obtained as thick viscous solutions having a dull yellow color.
Xanthan gum is an excellent and widely used suspending and viscosity building agent. Some of its particular uses are in oil well fluids, paint, sprays, and cleaning fluids. Xanthan gum, however, has a few disadvantages. It is very difficult to disperse and wet in water or brine so that hydration can take place. A high degree of shear is usually necessary in order to wet each gum particle. Once dispersal and wetting are accomplished, the hydration of the gum, as evidenced by the development of viscosity, is quite rapid. Xanthan gum and guar exhibit very different rheological characteristics, having different molecular configurations, and are obtained from entirely different sources. Various proprietary xanthan gums, having slightly different molecular structures and rheological properties by use of mutant strains of X. Campestris, are available from several manufacturers.
There is clearly a need for a stable dithionite hydrosol composition having such pseudoplastic properties that it is readily storable, even though subject to vibrations during tank car or tank truck shipment to a textile mill or to a pulp mill, for example, and readily pumpable when thereafter delivered to a storage tank for dilution to a solids content of 12-13% and short-term storage until needed, such as for bleaching textiles or woodpulp. However, attempts to use both guar gum and xanthan gum as suspending agents for sodium dithionite crystals have demonstrated that they have surprisingly unpredictable tendencies to form either gels or settled slurries, even during quiescent storage.